1. Field of the Invention
The invention relates to, and is based on, a method according to the precharacterising clause of the main claim.
2. Description of the Related Art
The so-called DVB-T standard describes a multi-carrier signal whose individual carriers which contain the user data are QAM-modulated, for example QPSK-, 16-QAM- or 64-QAM-modulated. The number of individual carriers which are used, and which are indexed by k, depends on the mode of the Fourier transform (FFT) which is used. In the 2 K mode, for example, k=0 to 1704, and in the 8 K mode k=0 to 6816. The totality of all the individual carriers transmitted within a fixed time range is referred to as a symbol, as shown by FIG. 1. Within this carrier range, some of carriers are 2PSK-modulated with additional information about the FFT mode which is used, the QAM order, etc., and these are referred to as TPS (Transmission Parameter Signalling) carriers.
Also present are many unmodulated individual carriers (pilots) which have a higher amplitude by the factor 4/3 compared with the mean square of the amplitudes of the aforementioned carrier types, and a rigid phase of 0° or 180°, as shown by the constellation diagram according to FIG. 2. The phase angle is determined by a pseudo-random number from a PRBS (Pseudo Random Binary Sequence). These pilots are referred to as continual pilots, they are found at fixed carrier positions and they are used for coarse adjustment of the frequency of the local oscillator in the receiver.
Likewise transmitted with increased amplitude and a rigid phase of 0° or 180° are the so-called scattered pilots. These are distributed uniformly over the carrier range and alternate their position within the carrier range from one symbol to the next, as shown by FIG. 1. Channel estimation at the carrier positions k is possible by interpolation of these scattered pilots along the frequency and time axes, and this information obtained by interpolation can then be used for channel correction of a perturbed DVB-T signal. With the aid of the constellation of one or more QAM-modulated individual carriers, it is possible to derive various transmission parameters which permit an assessment of the quality of the signal separately according to channel-specific parameters (e.g. signal-to-noise ratio) and influences caused on the transmission side (e.g. residual carriers or quadrature errors).
FIG. 3 shows the basic structure of a transmitter-receiver path of a system operating according to the DVB-T standard. In a data processing stage 1, the digital video signals to be transmitted are processed in the frequency domain and subsequently converted to the time domain (computer 2) by inverse Fourier transformation (IFFT). The I and Q components produced in this way are sent, after digitising in D/A converters 3 and amplification in amplifiers 4, to a quadrature mixture 6 in which they are mixed up to the desired output frequency with the components, mutually phase shifted by 90°, of a carrier signal 5 and are re-combined in an adder 7 and are broadcast.
In the receiver, on the input side, a back-conversion of the received input signal from the time domain to the frequency domain in turn takes place via FFT in the input stage 10, then the aforementioned channel correction is carried out by interpolation in a correction stage 11 with the aid of the scattered pilots, i.e. the received scattered pilots are corrected in such a way that, on the reception side, they respectively assume the ideal position according to FIG. 2 in the I/Q plane. In a subsequent signal evaluation device 12, the received video signals are processed further.
A DC component UI or UQ on the transmitter side, as schematically indicated in FIG. 3 by the adders 8, can lead to a perturbing residual carrier which shifts the constellation, in particular, of the central carrier of a DVB-T signal, as indicated in FIG. 3b (FIG. 3a is the ideal constellation of the central carrier). In the case of a multi-carrier signal of this type, the central carrier lies in the baseband at the frequency zero. In the 2 K mode, it lies at k=852, and in the BK mode it lies at k=3408. In the 2 K mode, this central carrier is modulated with user data and only sometimes constitutes a scattered pilot, so that the residual carrier power can be determined directly with reference to it, as is the subject of the prior patent application 199 48 383.3, our reference P 22751. In an 8 K mode, the central carrier is not modulated and sometimes constitutes a continual pilot or a scattered pilot. The residual carrier cannot therefore be identified directly with reference to it.